Why ancient organisms stored arsenic inside their cells

Earth’s earliest complex organisms endured harsh environments, and a new study reveals how these ancient life forms used arsenic to survive.

Researchers studied 2.1-billion-year-old fossils from Gabon’s Francevillian Basin. These fossils belong to eukaryotes, organisms with advanced cells.

The team found that these ancient organisms stored arsenic inside special cell compartments. This strategy helped them survive in seas with even low arsenic levels.

Using advanced X-ray technology, they mapped arsenic inside of the fossils. The analysis revealed that arsenic was not the result of later contamination. Instead, it was locked inside during the organisms’ lives.

Ancient seas triggered arsenic defenses

The study offers a fresh view of early life’s fight against toxins. The fossils lived in oxygen-rich seas with plenty of phosphate. However, even in these waters, arsenate – the toxic form of arsenic – posed a problem.

“The ability to cope with arsenic was not something eukaryotes developed randomly,” explained Dr. Ernest Chi Fru from Cardiff University.

During this period, Earth’s oxygen levels began to rise. This change made arsenate more common. Since arsenate closely resembles phosphate, it easily entered cells.

Organisms faced a tough challenge. They needed ways to avoid poisoning while still absorbing phosphate, a nutrient essential for life.

Fossils show arsenic stored in bodies

The fossils showed high arsenic concentrations, far above the levels in surrounding rocks. This suggests they absorbed arsenic during their lifetime.

X-ray imaging revealed arsenic at the core of tiny pyrite crystals inside the fossils. This points to a biological origin. The arsenic likely came from within the organisms, not from the environment after burial.

Other structures in the same rocks did not show such patterns. Abiotic concretions nearby lacked this arsenic core structure. Similar arsenic-rich patterns appear in some younger animal fossils. However, they are absent in purely mineral samples.

This difference suggests that the Francevillian organisms actively stored arsenic. It wasn’t simply absorbed from surrounding waters after death.

Arsenic storage for survival

The researchers tested three scenarios to explain arsenic in the fossils. First, they ruled out passive absorption from decaying organic material.

Second, the team dismissed the idea that sulfate-reducing microbes introduced the arsenic. The most likely explanation was bioaccumulation. These organisms absorbed arsenic during their lifetime and stored it inside their bodies.

“This suggests a pronounced biogenic control on arsenic storage,” wrote the researchers. They argue that arsenic storage was not just a survival trick. It was an adaptation tied to the rise of oxygen in Earth’s oceans.

Arsenic damaged ancient life systems

Arsenic disrupts life by mimicking phosphate. Once inside cells, it blocks key processes like energy production and genetic functions.

Modern organisms use efflux pumps or special enzymes to remove arsenic. Some even store it safely in vacuoles or vesicles.

The fossils showed traces of arsenic linked with other metals like nickel and copper. These metals can also be toxic in high amounts.

This suggests the organisms had a broader detoxification system. They may have stored multiple harmful elements together.

Modern sponges use similar storage

The researchers drew comparisons with modern sponges. These ancient animals often store arsenic in special compartments.

Sponges use this strategy because it helps them manage metal exposure from their filter-feeding lifestyle.

The Francevillian organisms might have done something similar. Their environment had low arsenic levels, but long-term exposure likely led to gradual buildup inside their bodies.

The stored arsenic later formed pyrite crystals during fossilization. The researchers’ model shows how this process left clear chemical markers in the fossils.

The evolution of complex life

The findings suggest that arsenic detoxification emerged very early in the evolution of eukaryotic life. It was likely not a rare or isolated event.

Instead, the ability to manage arsenic probably evolved multiple times throughout Earth’s long history, as different organisms adapted to shifting environmental conditions.

These arsenic-rich fossils reveal much more than just details about ancient organisms. They serve as powerful tools for researchers, offering a way to detect signs of early life in rocks that are billions of years old.

The chemical traces left behind provide strong clues about how life evolved and adapted in hostile environments.

Survival strategies of ancient life

Ultimately, the research highlights the importance of chemical signatures as records of biological processes.

Even when organic material disappears, these chemical patterns can tell the story of ancient survival strategies and environmental challenges.

By storing arsenic within their cells, these early organisms developed a unique way to survive in toxic seas. In doing so, they not only overcame dangerous conditions but also preserved a chemical fingerprint that allows scientists today to uncover their ancient existence.

The study is published in the journal Nature Communications.

Image Credit: Arnaud Mazurier and Abderrazak El Albani

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